Effect Of Black Carbon Radiative Heating On Cloud Microphysics Over Indo-Gangetic Basin

Airborne black carbon (BC), the most significant particulate absorber of solar radiation in the atmosphere, is an important contributor to both global and regional-scale climate forcing (Tripathi et al., 2005). In context of cloud microphysics, freshly emitted pure BC particles are hydrophobic (i.e., bad cloud condensation nuclei (CCN)). However, exposure in the atmosphere may transform BC to a hydrophilic state if these particles are coated with additional materials, such as sulfate and organic carbon (OC). In a recent study, Conant et al. (2002) has examined the effect of radiative heating of BC on the critical supersaturation spectrum of internally mixed aerosols. Two main uncertainties introduced in this work are due to lack of knowledge of actual state of mixing and realistic distributions of different aerosol species. Indo-Gangetic Basin (IGB) in the northern India is one of the most polluted regions in the world. The cloud microphysical processes in IGB are very complex and it requires an in depth investigation for understanding of the aerosol-cloud interaction in the region (Tripathi, et al., 2007). In the present work, an attempt has been made to study the effect of radiative heating due to BC particles coated with hydrophilic materials on cloud microphysics over IGB. For this purpose, we have used (a) a two-layer radiative parameter model based on Mie theory (Toon and Ackerman, 1981) to calculate the particle (monodisperse) absorption cross section; (b) a three-dimensional (3D) radiative transfer model, the spherical harmonics discrete ordinate method (SHDOM) (Evans,1998), which assumes a tropical continental atmosphere, to simulate the 3D spectral actinic flux over the study region; and (c) Extended Köhler theory (Conant et al., 2002) to simulate the effect the BC radiative heating on cloud droplet activation. The solar wavelength spectrum used ranges from 0.2 to 5 micrometer. Following the in situ measurements and modeling studies on mixing state (Dey, 2008) of aerosol species in IGB, we assumed that the absorbing BC core coated with purely scattering water-soluble sulfate, OC and/or a homogeneous mixture of sulfate and OC is a realistic choice. We have also used the realistic distributions of BC and other aerosols for IGB as modeled by Dey (2008) using aerosol measurements during ISRO-GBP land campaign II. The results relating the effect of BC radiative heating on cloud microphysics using realistic mixing state and distributions of aerosol species in a continental polluted environment over IGB will be discussed. References Conant, W. C., et al. (2002), Black carbon radiative heating effects on cloud microphysics and implications for aerosol indirect forcing, 1, Extended Köhler theory, J. Geophys. Res.,107(D21),4604. Dey, S., Aerosol radiative effects over Knpur region in the Indo-Gangetic basin, Northern India, Indian Institute of Technology, Kanpur, India, Ph.D. thesis, 2008. Evans, K. F. (1998), The spherical harmonics discrete ordinate method for three dimensional atmospheric radiative transfer, J. Atmos. Sci., 55, 429--446. Toon, O. B., and T. P. Ackerman (1981), Algorithms for the calculation of scattering by stratified spheres, Appl. Opt., 20, 3657--3660. Tripathi, S. N., et al. (2005), Aerosol black carbon radiative forcing at an industrial city in northern India, Geophys. Res. Lett., 32, L08802. Tripathi, S.N., et al. (2007), Aerosol indirect effect over Indo-Gangetic plain, Atmos. Env.,41,7037--7047.